Why standard tip angle is 118 degrees? 1

[pratyu]

I would like to know why "118 degrees" made the standard tip angle for a drill bit.Or for that matter, 135 degrees?

1. Is there anything called "standard tip angle"?
2. What factors determine the choice of the tip angle?

P.S: I posted the same question in Aerospace forum :

http://www.eng-tips.com/viewthread.cfm?qid=347077

 

[mcgyvr]

http://www.newmantools.com/machines/drillpoint.html

 

[Viktor]

118-120 is the standard for HSS drills. Nowadays leading companies use 140 for carbide drills.
A question "Why?" is another story - the above link does not provide clear answer although it rather simple but lenghly to explain with no figures.

 

[pratyu]

Viktor,
Well, I'm interested in the "Why" part of it. I have all the time for the story...

 

[Viktor]

Well..the story is as simple as....I will start to tell it in sections a bit later. To start, please think that what changes with the point angle: the direction of the chip flow with respect to the drill axis; the ratio "axial/radial" forces; the chip load on the major cutting edges (a.k.a. the lips) provided that the feed per revolution is the same; the rake and clearance angles of the chisel edge. The story is about balancing these factors for a given application (i.g. the work material, system rigidity, system runout, drill manufacturing accuracy,etc.), so it is rather simple and straightforward.

Viktor

http://viktorastakhov.tripod.com

 

[cloa]

Isn't it something like what's the average of 140 degrees and 98 degrees- two limiting cases- one being minimum force and other being maximum cutting of mild steel cutting with hardened steel probably HSS?

http://www.eng-tips.com/threadminder.cfm?pid=1529

Use translation assistance for Engineers forum

Note the rules include No Student posting

 

[Viktor]

New Post cloa (Petroleum)

Not even close. 140 is now the standard for carbide drills while 120 is still the standard for HSS drills. When you say "minimum force'? what the force you mean? When you say "maximum cutting" what is that? Being of great importance, the point angle is the second most neglected geometry drill parameter in the design/use of modern drills, particularly in high-efficiency drilling, e.g. in the automotive industry. Although it may sound unbelievable, there is a perfect explanation to this "medical" fact. I do believe that you can easily figure out this explanation if you recall what changes in the drilling systems were made over the last 15 or so years + in drill manufacturing including inspection + in grades/quality of drills' materials.

 

[hydtools]

I checked my book, Modern Machine Shop Practice, printed in 1888 and the tip angle is 120 degrees as determined by vendors such as Sharpe and Morse. According to the book text, Professor John E. Sweet presented a paper to the British institution of Mechanical Engineers describing tests he had done evaluating keeness and durability of the twist drill. Based on those test he concluded that the angle and grind geometry was appropriate as it produced proper sharpness and clearance while being durable. There may be other grind geometries for specialty work.

Interesting even way back then.

Ted

 

[Viktor]

Ted

You are right - it is established long time ago that 120 presents a balance. However a question remains: Why? An why modern carbide drills by leading tool manufacturers as for example Guhring and Mapal are supplied with standard 140?

Not that something has been changed since 1888 in terms of drills manufacturing and use. Nowadays, modern CNC drill point grinders as Walter and ANKA (for example) can apply ANY point geometry. Hydraulic and shrink-fit tool holder provide drill installation in the machine with minimal runout. Speeds and fees as well as machine power and axial force do not limit the application of drills of ANY design and geometry as it used to be in 1888. So the optimal point angle and the point geometry should be re-considered for these new conditions. A logical question is: WHY?

 

[cloa]

The answer is probably no abstract logic- only empirical trial and error determined the "best" angle. Sharpe and Morse created a whole bunch of different angled tips and cut mild steel with it and which one they felt gave the best result they went with that. Its unlikely these metalworkers passed any schooling higher than elementary school.

http://www.eng-tips.com/threadminder.cfm?pid=1529

Use translation assistance for Engineers forum

Note the rules include No Student posting

 

[Viktor]

cloa

Sharpe and Morse did a great job under conditions of the second half of the 19th century. However, "the best result" as vied today should be specified clearly. the best in terms of what - hole quality, productivity (the penetration rate0, cost per drilled hole, tool life and so on. "The best result" achieved on the manual drill press using hardened high-carbon steel drills may not be the same for:
- a modern high-speed CNC machine.
- modern HSS and carbide grades
- the modern drill manufacturing accuracy, i.e. lip height variation, web eccentricity, chisel edge centrality and so on.
- the ability of modern CNC drill measuring machines as for example Zoller Genius, Walter Helicheck Pro.

What does not change since their time is understanding of drill geometry and design as not much written on the matter.

 

[EdDanzer]

The machine tool and cutting tool industry is not an industry that accepts change quickly so standard methods and specifications continue long past their prime. Even adoption of new technology is slow in these industry in general.

Much of the work done by the pioneers of an industry was done by trial and error and is still relevent for average similar work so change may not be cost justifiable either. It maybe that the documentation of the orignal work was never published so there is no actual proof of the standard other than it works and is inexpensive to produce.

There are many special drill tip configurations availble for special production or material specific application if you can justify the cost.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[Viktor]

EdDanzer

In general, you are right. However, two points I would like to make.
First, as the conditions have been changed, the tool should be changed accordingly. This is not nearly the case.
Second, no need for cost justification. It is to say that when you grind a complicated point geometry manually, as it used to be for years, there is a significant additional cost. Nowadays, however, a CNC grinder is fully capable to reproduce virtually ANY point geometry with no or small 9if a special grinding wheel is needed) additional cost. The problem is that no many specialists left who know what to ask for.

 

[johnchrc]

We really need to pick some standard to use so we can design products. I might not know what type of drill is going to be used and don't care. Is this something I must now consider? Most of the time the angle is of no importance to the part design but with 3D modelling and having to add all the geometry to the CAD file I may need to know. This part of my job makes me pull my hair out. I guess in future I'll tolerance from 118 to 140 on the drill point angle.

- CJ

 

[EdDanzer]

Vikor,

We are in a lower margin low volume (1-20 parts)per lot job shop so the cost of the drill and sharpening cost can be very important. We do some carbide insert, carbide tip, solid carbide and special high speed steel drills for some applications. Most of the drills we inventory are 118° tip.

John,

I specify the maximum depth for the drill tip so it does not matter the type of drill used. The Komet insert drill we use have a semi-flat bottom so when I design a hydraulic manifold I have to consider how the tip will intersect with a cross passage or affect the next passage wall thickness during the design. In some cases I will specify the drill type for a hole to make sure things work out correctly in the shop.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[hydtools]

johnchrc, it is hardly worth pulling your hair out. Specify what you need. If the drill point shape does not matter, accept the CAD canned drill point which is usually 118 deg. There are more important issues in design than drill point unless it is important to the product design.

Ted

 

[johnchrc]

Ted,

Just an expression that belies growing questions. We now store all CAD models to the part's nominal dimensions. So, any manufacturing features that exist in the final model are included. We don't even have a machine shop and farm everything out so we have no standards except Machinery's Handbook or the CAD package default. If the machining work isn't in, or does not affect, the finished model than I don't care. I'll let the shop decide how they want to make the part. Otherwise I do care becasue the question will come backas an NCR without question. So, it is nuce to know that a 150 angle may be prefered and I can dimesion to include.

CJ



- CJ

 

[looslib]

Since drill point angle may vary, do not dimension the angle, but dimension the depth of the full diamater of the hole. The only time you need to worry (and pull your hair out) is if you have a limited space and do not want the drill tip to penetrate the bottom surface. In this case, a note saying drill shall not penetrate this surface would be sufficient.


"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

 

[Viktor]

[highlight #F57900]In this case, a note saying drill shall not penetrate this surface would be sufficient.[/highlight] - this is a new trend in ISO drawing standards? The drill may not penetrate a dozen of other surfaces - do I need to make a note for each? Be serious - there is no such a note in existence. Rather it should be clear from the part drawing!

About point angles. Obviously, nobody here wants to design/use high efficiency drill. It is Ok. In this case follow the industry standards as leading tool company do - 120 for HSS and 140 for carbide drill.

As for the drill with "a semi-flat bottom" - it is called sometimes a brad drill. Advantage - the short point so it is really useful what there is not much space for the drill is available at the exit. Tool life, shape of chip axial force etc. are 'bad' compare with a 'normal' 120-point drill.

EdDanzer (Mechanical)

If for you the cost of drill and cost of their re-sharpening are significant then pay attention to the quality of both. Try to track the number of re-sharpenings you get from each drill; use application-specific drills, etc. Never, and I mean it, NEVER use chip drills and poor re-sharpening (re-coating with no stripping). There are a number of simple rules to follow that can cut down your costs by a half easily.

 

[KENAT]

Sorry Victor but giving a minimum hole depth with "DO NOT BREAK THRU" is actually quite common practice in drawings to ASME Y14.100 as well. This may be in lieu of adding a drawing view of the far side or just as a clarification.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​